[mythtv-users] Keyframe identification in cutlist editor

[Nick Morrott]

On 22 February 2012 11:12, Mike Perkins <mikep at randomtraveller.org.uk> wrote:

> Having recently done some manual editing using avidemux, I can tell you that if
> your cuts are /not/ on keyframes the end result will likely be noticeable on
> screen as random blocks, hashes or other constructs.
>
> Makes sense really, only keyframes are complete frames, which means that when
> one partial frame (the one you cut on) is followed by another partial frame (the
> next one you cut on, but following a different keyframe) the difference
> information is too great to be resolved. Only when the /next/ keyframe comes
> along can the GPU/TV resolve everything correctly.

I use avidemux (after a pass through ProjectX) quite a bit and always
ensure the start of a new sequence starts on an I- (or key-) frame (as
any MPEG-2 stream Group of Pictures (GOP) should). Sequences end on P
or I frames, but these usually crop up in the right place anyway,
especially if the video fades to black over a second or two. I've not
seen any bad blocking artefacts as a result of not having /both/ the
start and end of a sequence fall on an I-frame - the starting I-frame
is the key one.

Some video editors (I don't think avidemux is amongst these) will
reconstruct affected GOPs after lossless cutting to ensure the
sequence of I,B and P frames at cutpoints allow successful
reconstruction of the video without blocking/distortion (which occurs
when a sequence of frame cannot be successfully rendered).

Cheers,
Nick

-- 
Nick Morrott

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